XAML or Extensible Application Markup Language (pronounced zammel [ˈzæmɫ̩]) is a declarative XML-based language created by Microsoft which is used to initialize structured values and objects.

XAML is used extensively in .NET Framework 3.0 technologies, particularly Windows Presentation Foundation (WPF) and Windows Workflow Foundation (WF).
In WPF, XAML is used as a user interface markup language to define UI elements, data binding, eventing, and other features. In WF, workflows can be defined using XAML.

XAML elements map directly to Common Language Runtime object instances, while XAML attributes map to Common Language Runtime properties and events on those objects.

XAML represents a bridge between the designer and developer teams.
A new role has emerged as the result of this fusion, that Paul Alexander, a technical program manager with IdentityMine, calls the integrator:

“The Integrator understands the needs of the developer while also supporting the needs of the designer to assure that the app’s UI is as compelling as it was designed, while also validating that the concepts can be realized in code from the developer.”

The integrator deals mostly with XAML code and provides an interface between the developer and designer, by structuring and modularizing the XAML.

The Designer<->Integrator<->Developer Model allows the design team to leave the XAML unattended and focus on having their assets effectively integrated into the project.
Designers can work with tools such as Expression Design, Inkscape or Adobe Illustrator and output the results as XAML.
The integrator then integrates the XAML into the project and passes it on to the developers, who need not to be concerned with design issues.
Obviously, this model also works perfectly well the other way around, and in some cases it is advisable to have the developer team establish the foundations of the project.Expression Blend, by Microsoft, makes this transition even easier, by accepting and generating XAML code that can be directly imported/exported from/to Visual Studio.